cmd: qfw: do not depend on x86

The qfw command interface used to depend on X86, this patch removes
this restriction so it can be built for sandbox for testing. For normal
usage, it can only be used with CONFIG_QEMU.

Signed-off-by: Miao Yan <yanmiaobest@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>